1. Field of the Invention
The invention relates generally to methods for analyzing molecular structures. Among other things, the invention relates to new methods for determining the three-dimensional structure of a target compound using nuclear magnetic resonance (NMR) crystallography and new methods for screening test compounds having three-dimensional structures similar to that of a target compound.
2. Background Art
Acquiring the three-dimensional structure of pharmaceutical solids stands as one of the greatest obstacles to the rapid development of new and targeted drugs. Current methods are plagued by lengthy research timelines (often measured in months and years) and inherent experimental limitations. For instance, x-ray crystallography relies on the ability to grow consistent and sizable crystals of a compound, powder diffraction methods require a “best guess structural starting point,” and computational methods are unreliable for all but the smallest, most rigid molecules.
Current NMR experiments allow access to virtually any feature of the nuclear Hamiltonian, i.e., dipole and quadrupolar coupling, chemical shift, etc. The nuclear Hamiltonian itself is so well characterized that ab initio quantum mechanical calculations are able to successfully compute NMR signals (such as chemical shielding and dipolar coupling) for a given molecular structure. Additionally, Solid-State NMR (SSNMR) has been shown to be a sensitive indicator of many structural features, including hydrogen bonding, stereochemistry, conformation, steric forces, and electrostatic interactions. Because of its sensitivity to structural features such as these, SSNMR has been employed during the last two decades to investigate a host of problems present in solids. This type of analysis became NMR crystallography when it was extended by three separate research groups in 2002 and 2003 to determine complete three-dimensional geometries (conformation) of molecular solids (the Griffin, Van Rossum, and Harper groups).
The first two research groups (Griffin and Van Rossum) studied similar molecules (peptide and protein, respectively) that had been isotopically labeled. See Rienstra, C. M. et al., PNAS 99:10260-10265 (2002) and Castellani, F. et al., Nature 420:98-102 (2002). They also used similar methods based on structural information gleaned from dipolar coupling. As with all SSNMR structural studies, they began by assigning the individual chemical shift values to corresponding nuclear sites in the molecule. Then, using dipolar coupling experiments at various mixing times (for both identical and dissimilar nuclear species), they were able to correlate both long range and short range atomic distances, and torsional angles. These introduced a series of inter-atomic constraints, which were used as a starting-place in the subsequent conformational search. Both research groups used a simulated annealing technique, a method that stochastically samples the space of allowed conformations. In this method, molecular potentials are configured to permit transitions among the multiple conformations consistent with the structural constraints. Griffin acknowledged that this method did not ensure that all regions of conformational space were sampled and thus employed a parallel technique to add additional rigor and certainty to his final result.
This second technique divided the search space into discrete nonoverlapping volumes and assigned each volume as allowed or disallowed, based on whether or not it contained viable structures. By eliminating structures that violated the NMR-imposed structural constraints, he found that there remained 56,975 allowed structures. Unlike Griffin, Van Rossum did not attempt to address the deficits of simulated annealing techniques (in particular, the lack of an exhaustive con formational search) and merely applied the method in order to find a solid-state magic angle spinning (MAS) NMR structure that satisfactorily described certain constraints he had uncovered.
Harper's method, published only months after Griffin and Van Rossum, was fundamentally different from the studies of those scientists. First, he used a small biomolecule (ambuic acid) at natural abundance, rather than using a large peptide that had been isotopically labeled to enhance the NMR signal. Like Griffin and Van Rossum, he began by assigning the 13C shift values to the appropriate nuclear sites. After this, Harper's method diverges from the other studies. See Harper, J. K. et al., J. Org. Chem. 68:4609-4614 (2003). Both Griffin and Van Rossum had used dipolar coupling as the NMR values of interest, but Harper used NMR chemical shift principal values (CSPVs) as the experimental foundation for his work. He used Alderman's FIREMAT technique to acquire the CSPVs for each nuclear position, and compared these with a series of calculated conformers. See Alderman, D. W. et al., Mol. Phys. 95:1113-1126 (1998). Ambuic acid is a relatively small molecule with a six membered ring as the central feature and two short sidechains. He searched through conformational possibilities by examining different structural features independently and creating a set of possibilities that he thought were “reasonable” in some instances (i.e., intramolecular hydrogen bonding conformations) and exploring the conformations of other moieties in the structure by rotating around bonds in 30 degree increments. Harper also explored possible intermolecular hydrogen bonding schemes in ambuic acid by hypothesizing the existence of a dimeric structure and calculating the shifts for the dimeric compound. He admitted that this analysis scheme was not an exhaustive conformational search and further wrote that the inclusion of all combinations of conformational changes was “avoided due to the great increase in number of computations heeded.”
After these NMR crystallography studies in 2002/2003, other research groups began to build upon these methods and to exploit structural features of molecules using SSNMR. The methods used by more recent NMR crystallography groups introduce subtle improvements to the methods proposed by those first three studies. However, all of these methods are tremendously expensive, not simply because of man-hours and experimental requirements, but most notably because of the overwhelming search of conformational space. The reason for this difficulty lies in the number of factors contributing to the NMR signal. Although single-crystal X-ray diffraction may enjoy a 1:1 correlation between real and reciprocal space, there is not an equivalent transformation from NMR that will yield a unique set of molecular coordinates. The researcher must therefore cull an enormous number of computationally generated polymorphs, compute theoretical NMR values for these, and contrast the theory with experimental NMR results. Obviously, the number of polymorphic possibilities increases exponentially with molecular size, ultimately prohibiting a complete conformational search. Although different methods have been attempted to reduce the total number of possible conformers, ultimately, NMR crystallographers must either submit to the systematic exhaustive search, limit themselves to the study of small or rigid molecules, or resign themselves to a high degree of inaccuracy in their results. Accordingly, current NMR crystallography techniques are iterative, costly, slow, and lacking in general application.
The pharmaceutical industry stands to benefit immensely should NMR crystallography become sufficiently accurate and robust so that it can be applied on a routine basis to characterize biologically relevant molecules. The impact on the pharmaceutical industry is further discussed below.
Distinct polymorphs of the same molecule may have significantly different solubility and thermodynamic stability. The thermodynamic properties of a given polymorph are extremely important to industrial manufacturing processes and storage. In one famous instance (the case of Abbott Laboratories' Norvir®), the manufactured formulation suddenly converted to a previously unobserved, more thermodynamically stable polymorph. The new polymorph was significantly less soluble (and less bioactive) than the previous form. This polymorphic “invasion” cost Abbott hundreds of millions of dollars trying to recover the first polymorph and an estimated $250 million in sales during the year the drug was withdrawn from the market. See Goho, A. “Tricky Business: The Crystal Form of a Drug can be the Secret to its Success” Science News 166:122-124 (2004).
Distinct polymorphs of the same molecule may have significantly different bioactivity. There are often distinct differences in bioactivity between two equivalently stable conformations. This means that the bioavailability and the overall biological activity of the drug in vivo may be different for two polymorphs of the same drug.
Thus, structural characterization is essential for pharmaceutical research and development. If molecular structures of investigational drugs could be consistently and accurately determined, it would significantly reduce the gap between the pace of discovery and development. Furthermore, it would also pave the way for greater throughput, and novel fields of research. Current methods create significant barriers of cost and time, discouraging all but the most promising of compounds from structural study. Removing this barrier will open up all biomolecules of interest to further study.
As mentioned above, burgeoning NMR crystallography methods are currently being used to determine molecular structure. Recent studies have shown NMR is capable of acquiring conformational features and even whole structures of samples that did not yield to analysis via other methods.
Effective drug research and design would benefit from a method that applies a combined SSNMR/computational approach in a swift and universally applicable manner. Thus, there is a need in the art for a fast, high-throughput method for determining the three-dimensional structure of a compound of interest. There is also a need, for example in rational drug design, for methods to quickly screen test compounds for those compounds that have a three-dimensional structure similar to that of a compound of interest.